Tabular silver halide grains are grains possessing two parallel crystal faces with a ratio between the diameter of a circle having the same area as these crystal faces, and thickness, being the distance between the two major faces, of two or more.
Tabular grains are known in the photographic art for quite some time. As early as 1961 Berry et al. described the preparation and growth of tabular silver bromoiodide graiins in Photographic Science and Engineering, Vol 5, No 6. A discussion of tabular grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, p. 66-72.
Early patent literature includes Bogg U.S. Pat. No. 4,063,951, Lewis U.S. Pat. No. 4,067,739 and Maternaghan U.S. Pat. Nos. 4,150,994; 4,184,877 and 4,184,878. However the tabular grains described herein cannot be regarded as showing a high diameter to thickness ratio, commonly termed aspect ratio. In a number of U.S. Pat. Nos. filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are described as e.g. U.S. Pat. Nos. 4,434,226; 4,439,520; 4,425,425 and 4,425,426 and in Research Disclosure, Volume 225, January 1983, Item 22534.
The anisotropic growth of the said tabular grains is known to be due to the formation of parallel twin planes in the nucleation step of the precipitation.
The shape of the tabular grains miy be variable: triangular, hexagonal, disc-shaped, trapezoidal and even needle-shaped grains can be formed. The said shape can be regular or irregular.
The appearance of triangular or hexagonal grains is mainly concerned with the number of twin planes: it has been observed that an uneven number of twin planes leads to a triangular shape of the grains, whereas an even number leads to a hexagonal shape, whereas the appearance of trapezoidal and needle-shaped grains is related with the coalescence phenomena or the formation of non-parallel twin planes. These topics have been discussed in J. Imag. Sci. 31, 1987, p. 15-26 and p. 93-99.
Emulsion preparation of tabular grains by means of the methods well-known by a person skilled in the art of photography leads to grain populations consisting of a mixture of all shapes of crystals described hereinbefore.
As a consequence many attempts have been made in order to improve the degree of homogeneity of the size and shape of the crystals. In this context EP-A's 0 566 076; 0 506 947; 0 518 066 and 0 513 722 and U.S. Pat. No. 4,797,354 are related with the preparation of monodisperse hexagonal tabular crystals. In said U.S. Pat. No. 4,797,354 the preparation has been described of tabular emulsions having a high percentage of hexagonal, tabular crystals, accounting for from 70 to 100% of the total projected area of the said crystals with an average aspect ratio of from 2.5/1 to 20/1. However the examples therein, and in the other references cited, are illustrative for a low yield of silver halide emulsion in the reaction viessel mixture, said yield being defined as amount of silver nitrate precipitated per liter of the said reaction vessel mixture.
For radiographic applications photographic advantages of tabular grains if compared with normal globular grains are a high covering power at high forehardening levels as set forth in U.S. Pat. No. 4,414,304. Further a high developability and high sharpness especially in double side coated spectrally sensitized materials can be obtained. The thinner the tabular grains and the lower the number of non-tabular grains in the total grain population the greater these advantages are. To express it in another way: a high degree of homogeneity in grain morphology is desired, leading to a high covering power in order to further offer the possibility to coat lower amounts of silver. With respect to ecology it is thus of utmost importance to prepare tabular grains rich in silver bromide having an enhanced covering power.
The desire to have morphologically homogeneous tabular crystals however doesn't match with another desired feature: a high degree of homogeneity requires preparation of tabular grains during a long time in diluted reaction vessels, which is undesirable from an economical (waste of time) as well as from an ecological (waste of preparation solutions) point of view. In order to manufacture emulsions in a cost-effective way the yield should be naximized, meaning a minimum end volume of the precipitation mixture for a maximum amount of precipitated silver halide. In U.S. Pat. No. 4,334,012 a suitable way has been disclosed of concentrating the reaction mixture volume in the reaction vessel by applying as well-known emulsion washing technique ultrafiltration in a continuous way during the precipitation steps. These references however do not include teachings with respect to the preparation of monodisperse emulsions.